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. Atmospheric pCO2 is a critical component of the globalcarbon system and is considered to be the major control of Earth's past,present, and future climate. Accurate and precise reconstructions of itsconcentration through geological time are therefore crucial to ourunderstanding of the Earth system. Ice core records document pCO2 forthe past 800 kyr, but at no point during this interval were CO2 levelshigher than today. Interpretation of older pCO2 has been hampered bydiscrepancies during some time intervals between two of the main ocean-basedproxy methods used to reconstruct pCO2 : the carbon isotope fractionationthat occurs during photosynthesis as recorded by haptophyte biomarkers(alkenones) and the boron isotope composition ( δ11B ) offoraminifer shells. Here, we present alkenone and δ11B -based pCO2 reconstructions generated from the same samples from the Plioceneand across a Pleistocene glacial–interglacial cycle at Ocean Drilling Program (ODP) Site 999. We finda muted response to pCO2 in the alkenone record compared tocontemporaneous ice core and δ11B records, suggesting cautionin the interpretation of alkenone-based records at low pCO2 levels. Thisis possibly caused by the physiology of CO2 uptake in the haptophytes.Our new understanding resolves some of the inconsistencies between theproxies and highlights that caution may be required when interpretingalkenone-based reconstructions of pCO2 .

climate of the past

Insensitivity of alkenone carbon isotopes to atmospheric CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; at low to moderate CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; levels